There is a strong need for reliable standards for spin concentration. Fusinite,a fraction (maceral) of naturally occurring coals, is one of the candidates. A fusinite sample isolated from a No. 6 coal from an Illinois mine was ground to 5fM particle size and then washed in boiled nitric/hydrochloric acids to remove paramagnetic metal ions impurities. Fusinite exhibits a single-line EPR spectrum related to organic radicals that have been stable over an historical period oftime. SQUID magnetometry of acid-washed fusinite confirmed the existence of st ates with S = 1/2 in the sample. Dependence of the spin density with temperature measured precisely by SQUID magnetometry was compared with EPR measurements. A standard sample designed with a specified number of spins can be prepared and precisely calibrated by SQUID magnetometry for use in EPR experiments.